An essential task which must be performed to obtain accurate measurements from displayed video images is the delineation of distances or areas to be measured. This task requires a determination of what portion of a delineated area is to be measured, calibration and indications for the measurements, the ability to accurately mark or draw what is to be measured on the screen and the ability to correct or "erase" all or portions of what has been drawn to correct mistakes, to calculate new parameters, or to make different measurements. For area measurements, this process is performed by physically indicating on the CRT screen the boundaries or outline of the area or areas to be circumscribed or filled in and measured. A critical factor in obtaining such measurements from television images is the speed and accuracy with which the operator can make the outline entry.
A user of such a system uses whatever implements are associated with the device which is to perform the actual calculation in the electronic apparatus. The conventional implement currently and conventionally in use is a light pen. Conventional light pen circuitry does not permit free hand drawing of the area. Points or "hits" are indicated on the screen and associated software translates the points into straight lines from point to point, or, based on the point indications, "draws" more sophisticated geometric shapes. Non-linear, or non-geometric, shapes are frequent in video displays of naturally occurring phenomena and, consequently, the conventional light pen requires a multiple number of accurately placed pen "hits" to create a linear or geometric approximation of the desired shape or area.
A first major drawback in the use of conventional light pens is the inherent characteristic which causes an interrupt signal to be generated for several electronic locations surrounding the actual pen tip. Another major drawback is in the propagation delay which causes the electronic position of the raw pen interrupt to appear to the right of the pen tip's physical location when positioned on a raster scanning monitor. Consequently, with conventional light pen circuits there is a need for software or hardware circuitry commonly associated with light pen interrupt processors. They function to enter the first interrupt into an address or other cell position indicator memory and, through a program of hardware or software control, decrement that location to a location more closely related to the physical position of the pen tip on the monitor. This is conventionally done with hardware or software which creates a subliminal blinking operation, detectable by the pen tip, to appropriately position the data point with the pen tip. When correctly located the location is then latched for subsequent entry into the video display memory device. After each point is accurately located, additional hardware and software is necessary to connect the points and process which type of of measurement is desired.
The remaining difficulty in determining area measurements is in transferring or translating the measurement and control of measurement and the record of the measured area location into a computer for further processing and storage, which conventionally requires even more processing hardware and software.
Such hardware and software is extremely expensive and is of limited flexibility in achieving an accurate and rapid drawing or marking function on a video display.